Wind power is in constant growth and is considered, among renewable energies, as the best candidate for becoming a real alternative to conventional, more contaminating power sources such as those derived from fossil fuels such as oil, gas or coal.
The increase in the number of wind facilities, and in consequence the number of wind generators, connected to the electricity network, causes integration problems which slow down said growth. Among these problems, the most important one is related to the behaviour of the wind generators faced by sharp changes in the network voltage, called voltage sags.
The wind generators which are currently most widely used are variable speed wind generators, whereby it has been achieved that the machines suffer less mechanically from gusts of wind that the electricity generated has fewer fluctuations and that greater use is made of the energy.
In particular, within this type of existing electric generators, either synchronous or asynchronous, to produce variable speed, the second one is being opted for due to the fact that synchronous generators have different drawbacks. One of the drawbacks of the synchronous generators is that all the power generated, prior to its supply to the electricity network, must be converted by electronic converters. Said converters should, therefore, be dimensioned to support the whole power of the wind generator, resulting expensive and voluminous. Its power losses also cause a decrease in the total output of the wind generator. In contrast, said drawbacks are improved with the asynchronous generator.
The most widely used asynchronous generator is the double-fed asynchronous generator wherein the stator winding is directly connected to the network whilst the rotor is connected to the network via a converter which permits controlling both the active and reactive power of the electric generator. Due to the fact that, the power which passes through the rotor is only a small fraction of that of the stator, the converters are smaller in cost and size, and generate fewer losses.
Although many of the features of the wind generators are improved with double-fed asynchronous generators, the robustness of the electricity generation facility is reduced due to the fact that said double-fed asynchronous generators are very sensitive to the faults that may arise in the electricity network, such as voltage sags. In particular, the power converter which is connected to the generator rotor is a very vulnerable part of the system due to the fact that when a voltage sag occurs in one or several lines, the current which appears in said converter may reach very high values, and may even destroy it.
This high current is produced during the voltage sags and is due to the demagnetization of the generator until this reaches the new state of magnetization corresponding to the voltage existing during said voltage sag. This transitory situation which occurs in the generator during the voltage sags generates an overvoltage, with the consequent overcurrent, which generates a flow in the rotor, hereinafter called free flow.
In normal conditions, the demagnetization of the generator is performed in the stator's resistance, where the magnetic energy is transformed into heat. In this way, the duration of the transitory is linked to the constant of natural time of the stator which is typically of the order of one or several seconds, the sufficient time to damage or even destroy the converter in the event of a voltage sag.
The typical solution to avoid the generator converter from being affected by these high currents which arise in the voltage sags consists of accelerating said demagnetization process and protect the converter from the associated overvoltages and overcurrents induced by said free flow.
To accelerate this process, fixed or variable resistances can be connected, in the stator or in the rotor and in series or in parallel, which reduce the demagnetization time, or, alternatively, the converter is used.
The most widely used technique, called crowbar, is based on the use of resistances of very low value, even reaching short-circuit, which are connected, using a thyristor bridge, in parallel with the rotor, if overcurrents are detected in the stator or in the rotor or overvoltages are detected in the rotor or in the DC bus. Nevertheless, this technique involves different drawbacks, such as due to the fact that the resistances used are of very low values, on the one hand, the demagnetization time continues to be considerable and, on the other, overcurrents are generated in the generator if the generator continues connected to the electricity network, which means that to avoid these overcurrents the network generator is disconnected and is not reconnected until the voltage returns to its nominal value. In this way, the converter is protected with this technique, but the generator is disconnected from the network, even through it may be for a short period of time.
An example of application of this technique is the system described in document WO200403019, which proposes including an electronic switch between the stator winding and the electricity network whereto it is connected, and a demagnetization element connected in parallel to either the stator or to the rotor. Said demagnetizing element is a variable resistance. In this system, if a sharp variation is detected in the voltage network, the generator is disconnected from the electricity network and the demagnetization element is connected. This element is controlled so that the voltage of the generator connectors is equalled, which is known as the magnetization status of the generator, with the new voltage of the network. With this control, the stator flow is set in a short period of time to a flow value which corresponds to the real voltage of the network, so that there exists a coincidence in the flow value and its phase between the induced voltage of the stator and the network voltage before the generator is reconnected to the network. If this is the case, once these two voltages have been equalled the generator is reconnected to the network and the demagnetization element is disconnected. This example, in addition to requiring the disconnection of the generator from the network would cause the continuous connection and disconnection of the demagnetization unit in the case of single-phase and/or two-phase voltage sags.
According to the state of the art, an objective of the present invention is to provide the known electricity generation installations from wind power with an alternative solution to the crowbar to control its behaviour in the event of voltage sags. The alternative solution should guarantee that the wind generator the installation comprises does not disconnect from the electricity network to which it is connected, overcoming the drawbacks that are derived from disconnecting it from and connecting it to the network in the event of voltage sags.
In particular, an object of the present invention is that the electricity generation facility converter is capable of withstanding the occasional situations of voltage sags without this being damaged and without needing to make use of the disconnection thereof.